1. Technical Field
This invention relates to the structure and preparation of an organic electroluminescent display device (abbreviated as organic EL display device, hereinafter) for use as a display device or light source.
2. Background Art
Display devices which use organic EL elements have the following sorts of advantages over the liquid-crystal displays that currently represent the mainstream in flat panel displays.
1) They are self-emissive, so the viewing angle is wider. PA0 2) Displays only 2-3 mm thick can easily be manufactured. PA0 3) A polarizing plate is not used, so the color of the emitted light is natural. PA0 4) The broad dynamic range in brightness results in a crisp, vibrant display. PA0 5) They operate over a broad range in temperature. PA0 6) The response rate is at least three orders of magnitude faster than liquid-crystal displays, easily enabling the display of moving images.
Despite such superiority, the appearance of organic EL display devices in the market was retarded for the following reason.
In general, organic EL elements include a stack of three thin films having different functions, an electrode in the form of a "transparent conductive film," an "organic layer including a light emitting layer," and another electrode made of a "metal or alloy having a low work function." Difficult problems arose in the manufacture of EL elements, since the "organic layer including a light emitting layer" and "metal or alloy having a low work function" are susceptible to degradation by moisture and oxygen, and the "organic layer including a light emitting layer" is readily soluble in solvents and less resistant to heat. Differently stated, in methods using water, organic solvents and heat, once the "organic layer including a light emitting layer" and a layer of "metal or alloy having a low work function" were formed, it was difficult to isolate and divide the elements. This means that when it is desired to manufacture an organic EL display device of an equivalent class to the currently available display devices realized with liquid crystal, the full-grown semiconductor manufacturing technology and liquid crystal display manufacturing technology cannot be applied without modification.
Under the circumstances, several techniques capable of separating second electrode elements without exposure to the ambient air were devised. With these techniques, it became possible to manufacture highly reliable organic EL displays.
One exemplary method is disclosed in JP-A 275172/1993, JP-A 258859/1993, U.S. Pat. No. 5,276,380, and U.S. Pat. No. 5,294,869. This method utilizes the phenomenon that as shown in FIG. 30, when walls 43 of a height exceeding the thickness of a film 44 constructing the organic EL medium are positioned between display line electrodes to be separated, and a material 41 for organic EL elements is vacuum evaporated from a direction not orthogonal to a surface of a substrate 33, the material 41 is not deposited in the areas shadowed by the walls 43.
However, in order to form light emitting lines of an equal width in satisfactory yields, insulators 42 having a greater width than the walls 43, called electrically insulative strips or pedestals 42, must be formed below the walls 43 as shown in FIG. 30. The reason is that since in the vacuum deposition process, the presence of the walls 43 obstructs the adhesion of an organic film in proximity to the walls 43, a structure without the insulative strips 42 permits short-circuiting to occur between the first and second electrodes in the proximity of the walls 43 where the organic film becomes thin. Then the manufacturing yield becomes extremely low with large sized substrates which make it difficult to improve the thickness uniformity of the organic film.
Inversely, when the insulative strips 42 are formed for the purpose of increasing the manufacturing yield, the width of light emitting lines is restricted by the region where the insulative strips 42 are formed. The width of the insulative strips 42 is designed in accordance with the angle between a metal evaporation source 31 and the substrate 33 and the size of the substrate 33 itself, as shown in FIG. 31, for example. More particularly, as shown in FIG. 31, for example, if the width of the insulative strip 42 is narrow, there arises the problem that the width of light emitting lines varies with the position on the substrate 33 because at position A where the angle between metal atoms traveling from the metal evaporation source 31 to the substrate 33 and the wall 43 is small, as shown in FIG. 32, a film is deposited without problems, but at position B where the angle between metal atoms traveling from the metal evaporation source 31 to the substrate 33 and the wall 43 is large, as shown in FIG. 33, the region shadowed by the wall 43 becomes wider so that the light emitting region becomes narrower. Accordingly, in order to produce light emitting lines of equal width over the entire area of the substrate, the insulative strips must be given a greater width including a margin. Also a margin of at least 3 .mu.m to 5 .mu.m is necessary for the alignment between the insulative strips and the walls when display devices are fabricated on a large sized substrate using an aligner of the full exposure type. However, widening the insulative strips directly incurs a reduction of the light emitting region, which is disadvantageous in achieving a bright display.
Alternative methods are methods of providing isolation between light emitting elements by furnishing cavity structures, trench structures or well structures, and forming light emitting elements in the respective structures, as disclosed in JP-A 262998/1996 and 264828/1996. Apparently, these methods give rise to a similar inconvenient problem.
Further, in methods of forming on an electrode insulative walls having inversely tapered structures, overhang structures or undercut structures as disclosed in JP-A 315981/1996, 283280/1997, and 161969/1997, insulative strips become necessary in actual practice from considerations to form light emitting lines of an equal width in good yields.